Fluid heating unit



1941- c. s; SMITH srm. ,2 82

I FLUID HEATI'NG UNIT Filed April 7, 1938 4 shet -sheet 1 29;! gfiarles5." mi

Oct. 7, 1941.

s. SMITH ET AL IFLU"ID HEATING UNIT Filed April '7, 1938OOOOQOOOOOOOOOOOOOOOOOOOOOO Char/e5 4 Sheets-Sheet 2 INVENTORS 5. Smz'ihgi CZz de B. Bax/er W ATTORNEY.

Oct. 7, 1941. c. 5. SMITH ETAL 2,257,821 FLUID HEATING UNIT Filed April'7, 1938 4 Sheets-Sheet s ig'i czar/536%??? I: In lyde B Bax/er Oct. 7,1941.

FLUID HEATING UNIT Filed April 7, 1938 q Fig: 6

- 4 Sheets-Sheet 4 INVENTORS Charles SSmi/z c. 5. SMITH ETAL- ,2

I'would weaken or oxidize the metal.

an initial temperature of more than l500 Irate of heat absorptionthroughout as is con- Patented Oct. 7, 1941 UNITED STATES PATENT OFF-ICEFLUID HEATING UNIT of New Jersey Application April 7, 1938, Serial No.200,598

22 Claims. 2(QL122-336) Uri 'perature of about 850 F., the air beingforced through the heater at a pressure of approximately 45 lbs/sq. in.with a maximum pressure drop of about 1 lb./sq. in, from inlet tooutlet. The usual forms of air heaters are unsuitable for suchconditions, being generally adapted for use only at low pressures or,where higher pressures are involved, are intermittent in their action,such as the air heaters used for heating blast .furnace air.Consequently-in order to meet the imposed conditions, this inventionemploys an'air heater wherein the air passages are formed by tubes whichhave their ends connect sure vessels, the hot heating gases con ed topres- H tacting the 5 tubes exteriorly in such a manner as to insure themaximum air heating capacity while preventing metal temperatures fromrising to values that In order to raise the temperature of the air toapproximately 850 F. the heating gases may have F. and the inventionprovides means to insure that under such conditions, the temperature ofthe tube M metal does not exceed a safe working value for the materialused. This involves a continuous progressive movement of the fluidinside all of the tubes and the maintenance of a sufficient massvelocity of flow through each of the tubes, in relation to local gastemperatures and local gas velocities outside, or to the intensity ofthe heating source, to prevent overheating of the tube metal at anylocation.

An additional object, therefore, is in the v arrangement of tubularheating surface whereby the flow of fluid through the tubes isprogressive from inlet to outlet and preferably in a generallycounter-current direction with respect to the flow of heating gases;other objects being to establish suitable relations between thevelocities of air and heating gases, and to maintain as high a sistentwith the relation of metal temperatures to temperatures of the gas andair.

Further objects contemplate the protection of illustrated.

portion of the tubular heating surfaces from over-heating, due to highfluid temperatures within the tubes. and high gas temperatures without,by causing the inside fluid to flow through a selected hightemperatureportion of the fluid heater at a higher mass velocity than through otherportions; by maintaininga lower mass velocity of the outside fluid overa high-temperature portion than over other portions; and by restrictingthe amount of radiant heat absorption, Such protective features are ofparticular importance when air is the fluid being heated, as thetendency for oxidation at high temperatures is greater than with otherfluids.

The invention also includes'the arrangement of heating furnace and meansassociated therewith for recirculation of combustion gases whereby thetemperature of the heating gases may be controlled to prevent injury tothe tubes, and whereby the high efiiciency of operation is unimpaired,and the desired heating effect may be secured at reasonable cost.

Additional features will appearfrom the description which follows,particularly when read in conjunctionwith the accompanying drawings inwhich selected embodiments of the invention are In the drawings:

Fig. 1 is a sectional side elevation showing an embodiment of theinvention in an air heater of the direct or separately fired type;

Fig. 2 is a vertical section along the line 2-2 of Fig. 1;

Fig. 3 is a vertical section along the line 3- of Fig. 1 showing detailsof the furnace;

Fig. 4 is a sectional plan along the line 44 of Fig. l;

Fig. 5 is a sectional plan along the line 55 of Fig. 1; and

Fig. 6 is a sectional side elevation similar to Fig. l but showing amodification.

The arrangement in general comprises a furnace I having its combustionchamber 2 provided with suitable firing means such as one or more oil orgas burners 3. The hot gases of combustion are conducted from thechamber 2 through the connected upright passages or flues 4 and 5,flowing upwardly through the flue 4 and downwardly through the flue 5,and being discharged from the setting through the gas outlet 6. Thetubular heating surface as shown includes the banks of tubes 1 and 8which are so arranged that a the total heating surface lies within eachof the flues 4 and 5, the tubes of each bank having upright portionsextending across the gas passage which connects the flues. The air orother fluid to be heated, is delivered to the inlet header 9 underpressure through the inlet opening I at one end and is caused to flowthrough the tubes 1 from header 9 at their lower ends to an intermediateheader II at their upper ends, the tubes 1 extending longitudinally ofthe upright flue and being spaced apart .in rows throughout theirlengths, as indicated-in the sectional views Figs. 4 and 5.

The tubes 8 form a substantially L-shaped bank in which the tubes arebent intermediate their ends as at I2 to provide upright leg por-' tionsl3 which are connected at their upper ends to the intermediate headerI:I, -and horizontally disposed leg portions M which are connected attheir outer ends to an outlet header l5 having an end opening l6 fordischarge .of the heated Thus, the tubes I and 8 provide parallel pathsfor the flow .of .air within the respective banks, and in flowing firstthrough the tubes .I and .then through .the tubes 8,, the air flows in adirection generally ,countercurrent to the flow of the heating gases..Since the heating gases follow an inverted .U-shaped flow path, theportions I4, 13 of the tubes .8 are .contacted successively, and theflow of gas is transverse to the .tube lengths comprising each .portion.In their downward flow through the .flue .5, the heating gases sweep thetubes TI longitudinally.

The tubes .8 in each -of the vertical rows II, I8, for example, are bentas at [9, 2.8 to form a plurality of horizontal rows 2|, .22 wherein.the tubes are staggered and more widelyspaced than tubes in upper rowsof the bank. An additional row of tubes 23 between the headers 1.] andI5 formsa protective screen for the front Wall .24 and end wall 25 atthe .upper portion .of the flue ,4, so that radiation of heat from .therefractory walls, which would otherwise be active, ,is largely .orwholly neutralized.

A row .of tubes 26 .extends upwardly from the inlet header 9 within theflue ,5 adjacent the outer face of bridge wall 2-I, the tubes .26 beingbent at 28, 29 above the bridge wall extension 38 into a plurality ofhorizontal rowssuch as 3|, 32 wherein the tubes are staggered atrelatively wide spacings, similar to the tubes the upper succeeding rows2|, 2-2. The tubes .26 are connected .at their outlet ends to the outletheader I5 and thus form a path for fluid flow .from the inlet header 9which bypasses the intermediate header and affords a lower flowresistance as compared with other tubes. Consequently, more air isconducted through the tubes 26 than through other tubes, and since theair is of lower temperature than the air flowing through the tubes 8,their horizontal portions BI, 32 serve ited amount of radiant heat,partly by reflection from :the walls .of the flue and partly by .radia-.tion from the hot .gases vin the space above the arches .33 and 3d.

The bathe-3,5 projects upwardly from the bridge wall extension 30 andbeing spaced .at .its .upper :end from the intermediate header -.l I,directs the v-,1 ieati ggases across the upper portions of tubes 8 inpassing to the flue 5 where the gases flow in contact with the uprighttubes 1.

In this embodiment, the tubes I, 8 and 26 are of equal diameters andspaced at equal distances across the width of the setting except in thelower horizontal rows 2!, 22, 3|, 32 where the spacing of tubes isincreased as already described. However, the Spacing .between rows isprogressively decreased in the direction of gas flow, thus reducing theefiective gas flow area :as the gases become cooler and maintaining as.higharate of heat absorption throughout as will not cause overheatingof the tube metal; the spacing between rows being a maximum for the rowscontaining the horizontal portions of tubes 8 and 26, an' in-termediatespacing for the rows containing the vertical portions I3 of tubes 8, anda minimum spacing for the rows of upright tubes I.

The tubes 8 and 26 in the lower rows 2|, 22, and 3|, 32-, respectively,where the gases .are hottest, are widely spaced so that while the heatreceivedby radiation is relatively high, the heat transfer by convectionis relatively reduced, in order to keep the metal temperature more closeto that of the inside air. "However, for thesucceeding rows where thegases are cooler and there is little or no radiation, the tubes 8,including the vertical portions I3, are more closely spaced in order toprovide a high rate of heat absorption by convection without overheatingthe tube metal. In the flue 5, the spacing of tubes 1 is further reducedto increase the mass velocity andheat transfer rate, which here is safeand proper, ,due to the fact that the gases are lower in temperature andflow of gas is longitudinally of the tubes I instead of across the tubesas in the case of tubes 8 and 26.

The tubes in the flue 4 are exposed to the hottest gases and sincetheflow of gases is transverse to those tubes, the heat transfer betweenthe gases and tubes by convection is higher than .for longitudinal flow,and a correspondingly high mass velocity of fluid flow through suchtubes helps to prevent the metal of the tubes from be- .comingoverheated. Accordingly, the mass ve- .of tubes 8 connecting the headersII and I5.

Due to the lower flow resistance of tubes 26, a still greater massvelocity is maintained through these tubes to further protect thoseportions which extend across the flue 4 and are initially traversed bythe hottest gases.

.Since the fluid in the tubes 8 is at its highest temperature and thesetubes are exposed to the hotter gases, the tubes 8 are preferably formedof a metal having higher heat resisting characteristics than the tubes Iwhich are in a zone of cooler heating gases and carry the inside fluidat lower temperatures. .Consequently, the use of an intermediate headerII is advantageous in connecting the banks of tubes 1 and 8 whendifferent metals are used, and when the numbers of tubes in the banksare different.

The gas outlet 6 is preferably subjected to an induced draft byconnection to a fan 36 for example, as shown in Fig. 6, and a portion ofthe gas discharged from the fan is returned to the combustion chamberand adjoining passages for downwardly of the gas being dischargedthrough a suitable flue 31, having a damper 38, to a stack (not shown).

Opposing side walls 39, 40 of the furnace have hollow interiors 4|, 42which are connected at the bottom by the hollow tile 43 disposed beneaththe furnace floor. A duct 44 leading from the discharge side of theinduced draft fan is connected through a branch 45 to a duct 46paralleling the furnace along one side and a passage 41 connects theduct 46 with the bottom of the side wall space 4|, a portion of thegas'from passage 41 being directed through the tile 43 to the oppositewall space 42. The amount of gas admittcd through the passage 41 iscontrolled by means of damper 48. The walls are provided with openings49, 50 for the discharge of gas from the spaces 4|, 42 into the upperportion of the combustion chamber. The openings 49, 59 are arranged inspaced groups along each side wall, one group comprising openings 49discharging directly below the main arch and another group comprisingopenings 5|] discharging below the outlet passage 52 through which theheating gases flow from the furnace into the flue 4.

The side wall space 4| to which the returned gas is first admitted, isextended vertically as at 53 to provide communication through one ormore openings 54 with the enclosed space 55 above the furnace arch 5|.The gas admitted to space 55 from the wall space 53 is discharged intothe combustion chamber through the openings 56. In order to equalize theflow of gas through the side openings 49, 59 in opposite walls, theopenings 54 above the furnace arch are at one side only to compensatefor the greater flow resistance for the opposite side where the gas isrequired to travel through the hollow floor tile 43.

A connection 51 conducts gas from duct 44 to the hollow interior 58 ofthe bridge wall 21, from whence it is discharged through the rows ofopenings 59 into the stream of hot gases leaving the combustion chamber2 through the outlet passage 52. A damper 69 in the connection 51controls the amount of gas recirculated in thi manner.

The arrangement of the gasports 49, 50, 56 and 59 as well as thedirection of discharge therethrough, is advantageous in promotingthorough is a minimum of interference with combustion of the fuel.

The gases are introduced at a plurality of locations in intersectingdirections and, being admitted through relatively small ports,

form jets which penetrate the body of hotter gases to produce anintimate mixture of lower and substantially uniform temperaturethroughout the stream of heating gases. The jets from the wall ports 49enter the combustion chamber from opposite sides and the jets from thearch ports 55 enter in a downward and intersecting direction; the jetsfrom the ports 50 also enter from opposite sides and intersect thestream of heating gases adjacent the outlet from the chamoer; and at asucceeding location, the jets from ports 59 penetrate the gas stream ina direction transverse to the jets from the ports 50. In this manner,maximum effectiveness of recirculation is insured, and the desiredtempering action obtained, with the least amount ofv gas being returned,thus reducing the load on the fan and maintainingefficiency as high aspossible under the working conditions.

There is also a cooling action on the furnace walls due to the admissionof relatively cool gases through the furnace arch 5| and beneath thesame, to provide a constantly moving layer of circulated gases whichserve to protect the arch 5| and front wall 6| from radiant heat of theburning fuel. The bridge wall 21 is also maintained at a safe workingtemperature due to the circulation of cooled gas through the interiorspace 58. In addition to the thorough mixing of recirculated gases withthe fresh gases within the combustion chamber, there is a further mixingaction afforded by the admission of flue gas through the openings 59 andby the arrangement of arches within the flue 4, which being offset andat different levels as at 33, 34 deflect and mix'the heating gases inadvance of their contact with the air heater tubes.

In the modification shown in Fig. 6, there is the same generalarrangement of furnace and connecting fines, as well as means fortempering the heating gases, as in the embodiment hereinbeforedescribed; consequently, the same reference characters have been appliedto parts which correspond. The heating surface, as before, is dividedbetween the two flues 4, 5 and provides a greater velocity of flowthrough tubes 8 in the hotter zone than through tubes 62 in a coolerzone. The tubes 62 are bent back and forth at successive levels 'to formcontinuous lengths of return bend tubes between the inlet header 9 andthe intermediate header II. The

tubes of one row 63 have portions lying adjacent the end wall25 and rearwall 64 of the flue 5 where the gases turn to flow downwardly. Bothgroups of tubes 8, 62 have vertical portions I3, 65 respectively,extending across the gas passage connecting the fines above the bridgewall extension 30, and the flow of air from inlet header 9 is generallycounter flow throughout with respect tothe heating gases, the airflowing in parallel through tubes 62 from inlet header 9 to intermediateheader H and thence through tubes 8 to the outlet header I5.

In the embodiment shown in Fig. 1, the intermediate and outlet headers Hand I5, are supported respectively on structural members 66 and 61,while the inlet header 9, at the lower end of flue 5, is suspended fromthe intermediate header by means of the upright tubes 1. The inletheader has a bearing member 68 near each end which is in slidingengagement with the frame plate 69 and is thus permitted to move in avertical plane to allow for expansion and contraction of the tubes 1.The sliding seals 10 at oppositesides of the outlet casing preventleakage of gas from the setting during movement of the header 9. In Fig.6, the support for headers I and I5 may be the same as in Fig. 1, andinlet header 9 may be fixed in its relation to the intermediate headersince the tubes 62 which connect headers 9, due to the plurality ofbends therein, will fully compensate 'for expansional and contractionalvariations in length.' i

It is to be understood that other forms and embodiments are permissiblewithin the scope of this invention, and with respect tosome of thefeatures, and for certain purposes, the fluid to be heated may be liquidor gaseous, or may be a mixture of a liquid and its vapor. Consequently,it is intended that the definition of invention as set forth in theappended claims,

be broadly interpreted and. made subject only to such restrictions asare specifically expressed or imposed by prior art.

We claim:

1. A fluid heater comprising means forming first and second uprightfiues and means forming a gas passage connecting said flues at theirupper ends, a bank of tubes within each flue and eachbank includingtubes having portions extending upwardly across said passage,fluidpressure means maintaining a progressive flow of fluid through thetubes of each bank in succession, means maintaining a flow of heatinggases in contact with said tube in substantially countercurrent relationto the progressive flow offluid therethrough, and means for increasingthe mass velocity of said gases and of said fluid in the respectivedirections of flow.

2. In a fluid heating unit, means forming adjacent first and secondupright flues connected at their upper ends for serial flow of heatinggasestherethrough in an inverted U-shaped path, a bank of tubes withinsaid first flue so formed and arranged relative to the path of heatinggases as to cause said gases to traverse different portions of the bankin succession, a bank .of tubes within said second flue, means admittingfluid to the lower ends of the tubes in; saidsecond flue, meansconnecting said banks of tubes at their upper ends, fluid-pressure meansfor causing said fluid to flow in series through the tubes of bothbanks, and means for increasing the mass velocity of said gases and ofsaid fluid inthe respective directions of flow.

3. In a fluid heater comprising means forming adjacent upright flueshaving a connection at their upper ends for the flow of heating gasestherethrough in an inverted U-shaped path, a bank of tubes havingportions extending horizontally across one flue and other portionsextending vertically across said connection, a bank of tubes extendinglongitudinally of said adjacent flue, means forming a fluid flowconnection between the upper ends of the tubes of said banks, means forcausing said gases to flow along said longitudinally extending tubes ata greater mass velocity than across said horizontally and verticallyextending portions, and means for causing fluid to flow in seriesthrough the tubes of said banks.

4. In a fluid heater comprising tubes forming a; bank havirg angularlyrelated portions and means directing a flow of heating gasessuccessively acrosssaid portions in transverse relation to the tubesthereof, said tubes being disposed in rows extending transversely of theflow of gases and including tubes more widely spaced in said rows in theportion first contacted by said gases than in a subsequently contactedportion.

5. In a fluid heater comprising tubes forming a bank having angularlyrelated portions and means directing a flow of heating gasessuccessively across said portions in transverse relation to the tubesthereof, said tubes being disposed in rows extending transversely of theflow of gases and including tubes bent intermediate their lengths: toform a greater number of rows in one portion than in an angularlyrelated portion, said greater number of rows being arranged forwardly ofthe portion first contacted by said gases and providing a greater areafor the flow of gases across said rows than across any succeeding row ofsaid bank.

6. In a fluid heater comprising tubes forming a bank having angularlyrelated portions and means directing a flow of heating gases successively across said portions in transverse relation to the tubes thereof,said tubes being disposed in rows extending transversely of the flow ofgases and including tubes bent intermediate their lengths to provide agreater tube spacing in rows first contacted by the heating gases ascompared with the tube spacing in rows subsequently contacted.

7. Fluid heating apparatus comprising connected banks of tubes, thetubes of one bank having a greater combined internal cross-sectionalarea than the tubes of a connected bank, means directing a stream ofheating gases first over the tubes of smaller area and then over thetubes of larger area, and means causing a fluid to be heated to flow inparallel through the tubes of each bank and successively through thetubes of said connected banks in countercurrent relation to the flow ofheating gases, said gases being directed transversely of tubes firstcontacted and longitudinally of tubes last contacted at mass velocitiesincreasing to a maximum for said longitudinal flow.

8. Fluid heating apparatus comprising means forming first and secondupright flues and means forming a gas passage connecting said flues attheir upper ends, a bank of upright tubes extending longitudinally ofsaid second flue and across said passage, an L-shapedbank comprisingtubes having portions extending across said first flue and otherportions across said passage, said up right tubes providing a greatercombined fluid flow area than the tubes of said L-shaped bank, meansforming a fluid" flow connection between the tubes of both banks attheir upper ends, means directing heating gases through said flues incontact with all of said tubes, and means admitting a fluid to be heatedto the lower ends of said upright tubes and causing said fluid to flowsuccessively through said upright tubes and through the tubes of saidL-shaped bank in a direction generally counter to the flow of heatinggases.

9'. In a fluid heater comprising an inlet and an outlet header, meansincluding tubes forniingv separate fluid" flow paths connecting saidheaders each having portions of the tubes there'- of arranged in rows,means forcing fluid through said tubes under pressure from said inletheader, means directing a stream of heating gases successively over saidportions, and means whereby the fluid carried by tubes in at least oneof the rows first traversed by said heating gases is maintained separatefrom the fluid carried by tubes in other rows and heated toa lowertemperature than the fluid' carried by tubes in rows subsequentlytraversed.

10; A fluid heater comprising tubes bent to form horizontally andvertically extending leg portions, means for directing a flow of heatinggases transversely of said horizontally extending portions andsubsequently over said vertically ex* tending portions, fluid-pressuremeans for causing fluid to flow through said tubes, and means formaintaining the fluid in tubes whose horizontally extending portions arefirst contacted by said gases at a lower" temperature than the fluid intubes whose horizontally extending portions are subsequently contacted.

11. In a fluid heater, a plurality of tubes arranged in rows and havingportions disposed in intersecting planes, means directing a stream ofheating gases successively over said portions, means connecting thetubes in all of said rows at the end of at least one of said portions,and

means causing fluid to pass through portions of the tubes firstcontacted by said gases at a higher mass velocity and lower temperaturethan the velocity and temperature of fluid passing through correspondingportions of tubes subsequently contacted. I

12. In a fluid heater having first and second upright flues and meansforming a gas passage connecting said flues at their upper ends, tubeswithin each of said flues and having portions ex' tending upwardlyacross said gas passage, a header connecting said tubes at the upperends of said .portions, the tubes in said first flue having portionsdisposed in successive horizontal rows, means for causing heating gasesto flow through said flues in transverse relation to said rows andupwardly extending portions, contacting first said horizontally disposedportions and subsequently said upright portions, means causing a fluidto flow through said tubes under pressure, and means maintaining thefluid in the lowermost of said horizontal rows at a lower temperaturethan the fluid in succeeding rows.

13. In a fluid heater having first and second upright flues and meansforming a gas passage connecting said flues at their upper ends, aninlet header at the lower portion of said second flue, an outlet headeradjacent an outer wall of said first flue and an intermediate headeradjacent said passage, tubes within said flues having their lower endsconnected respectively to said inlet and outlet headers and their upperadjacent ends connected to said intermediate header, and other tubeswithin said flues connected at their opposite ends to said inlet andoutlet headers, means causing heating gases to flow successively throughsaid flues in initial contact with said other tubes, and means admittingfluid to said inlet header at a pressure sufiicient to maintain aprogressive unidirectional flow of fluid throughout all of said tubesfrom said inlet to said outlet header.

14. In a direct-fired fluid heater in combination with its furnace, anupright flue having gas inlet and gas outlet openings at its oppositeends, said flue having its inlet opening in communication with thecombustion chamber of said furnace for the admission of heating gases tosaid flue, a bank of fluid heating tubes within said flue intermediatesaid inlet and outlet openings, means whereby gas discharged from saidoutlet opening is combined with said heating gases prior to theircontact with said tubes, and an interrupted baflie formationintermediate said gascombining means and said tubes comprising offsetportions at different levels.

15. In a direct fired fluid heater, in combination with its heatingfurnace having fuel firing means adjacent one end and means forming anoutlet for heating gases from its upper portion adjacent its oppositeend, fluid heating tubes and means directing heating gases from saidoutlet over said tubes whereby the temperature of said gases is reduced,and means for mingling a controllable portion of said cooled gases withthe body of heating gases prior to discharge through said outlet, saidcooled gases being directed into said heating gases in jets penetratingsaid heating gases in intersecting directions adjacent each end of saidfurnace.

16. In a direct fired fluid heater, in combination with its heatingfurnace having walls of hollow construction, inlet and outlet fluesconnected for sequential passage of heating gases from said furnace,fluid heating tubes in the path of said gases within said flues andconnected for progressive flow of fluid therethrough under pressure incounter-current, relation to said heating gases, and means forcontrolling the temperature of said heating gases comprising connectionsbetween said outlet flue and said hollow walls and means for directingcooled gases from the interiorsof said walls into the heating gasesprior to contact of said gases with said tubes, said means beingarranged to direct the cooled gases into the heatinggases in a pluralityof jets in intersecting directions adjacent at least one of said walls.I

17. In a fluid heater comprising a bank of spaced tubes arranged insuccessive rows and having a common connection at one end, the tubes inone or more rows constituting a group of tubes different from a group inone or more other rows, means for directing heating gases over saidgroups of tubes in succession, means for directing fluid under pressureseparately through tubes of the respective groups toward said commonconnection, and means for maintaining the fluid directed through tubesof a group first contacted by said gases at a lower temperature than thefluid directed through tubes of a group subsequently contacted.

18. In a fluid heater comprising a bank of spaced tubes arranged insuccessive rows and having a common connection at one end, the tubes ina plurality of adjacent rows constituting a group of tubes differentfrom a group in a plurality of other adjacent rows, means for directingheating gases over said groups of tubes in succession, means fordirecting fluid under pressure separately through tubes of therespective groups toward said common connection, and means formaintaining the fluid directed through tubes of a group first contactedby said gases at a lower temperature than the fluid directed throughtubes of a group subsequently contacted.

19. A fluid heater comprising means forming adjacent first andsecond'flues having a connection for serial flow of heating gasestherethrough, a bank of tubes within each flue and at least one of saidbanks including tubes having portions extending across said connection,fluid pressure means for maintaining a progressive flow of fluid throughthe tubes of each bank and through said banks in succession, means formaintaining a flow of heating gases in contact with said tubes insubstantially countercurrent relation to the progressive flow of fluidtherethrough, and means for increasing the average mass velocity of saidgases and of said fluid in the respective directions of flowsubstantially throughout the extent of said flues and banks.

20. A fluid heater comprising means forming first and secondsubstantially parallel flues and means forming a gas passage connectingsaid flues at their adjacent ends, a bank of tubes within each flueincluding tubes having portions extending-across said passage,fluid-pressure means maintaining a progressive flow of fluid through thetubes of each bank in succession, means maintaining a flow of heatinggases in contact with said tubes in substantially countercurrentrelation to the progressive flow of fluid therethrough, and means forincreasing the mass ve-' locity of said gases and of said fluid in therespective directions of flow.

21. In a direct fired fluid heater, in combination with its heatingfurnace having fuel firing means adjacent one end and means forming anoutlet for heating gases from its upper portion adjacent it oppositeend, fluid heating tubes and means directing heating gases from saidoutlet over said tubes whereby the temperature of .said gases isreduced, and means for mingling a controllable portion of said cooledgases with the bodyof heating gases prior to discharge through saidoutlet, said cooled gases being directed into said heating gases in jetspenetrating said heatinggases in intersecting directions adjacent oneend of said furnace.

22. In a direct fired fluid heater, in combination with itsheatingfurnace having fuel firing means adjacent one end and meansforming an outlet for heating gases from said furnace adja-v cent itsopposite end, fluid heating tubes and means directing heating gases fromsaid outlet over said tubes whereby the temperature of said gases isreduced, and means ior mingling a controllable portion of said cooledgasesw-ith the body of heating gases prior to discharge through saidoutlet, saidcooled gases :being directed into said heating gass in jetspenetrating said heating gases in a downward direction adjacent thefuelrfiring end of said furnace.

' CHARLES S. SMITH.

CLYDE B.

